Native tree root exudates promote tolerance of simulated herbivory of an invasive tree via altered functional traits

Aims Invasive plants often succeed despite attack by generalist herbivores in the introduced range. Whether invasive plants tolerate herbivory by adjusting functional traits, modulating the rhizosphere bacterial community, or both, and how root exudates of native plants influence these tolerance mechanisms, is yet to be tested. Methods We examined the impacts of defoliation (simulated herbivory) and root exudates from a native tree Ailanthus altissima on the growth, functional traits, and rhizosphere bacterial community composition of the invasive tree Rhus typhina. We conducted a greenhouse experiment with three levels of simulated herbivory (0%, 40%, and 80% of leaves removed) and four A. altissima root exudate treatments (none, weakened with activated carbon, natural, and added). Results In response to defoliation when grown in monoculture, R. typhina decreased aboveground biomass allocation but increased specific leaf area, whereas rhizosphere bacterial community composition was unaffected. When grown in mixture, defoliation decreased total biomass in the weakened root exudates treatment but not with natural and added root exudates, suggesting that A. altissima root exudates reduced R. typhina biomass but increased R. typhina tolerance of defoliation. Ailanthus altissima root exudates also reduced the negative impact of defoliation on putative keystone bacteria relative abundance. Conclusions Our results demonstrate that root exudates of native plants can promote tolerance of invasive plants to simulated herbivory via influencing functional traits and potentially also rhizosphere keystone taxa. Our study adds to evidence that plasticity of functional traits and the rhizosphere bacterial community may contribute to invasive plant tolerance of biotic stressors.